It is known an engine for an automobile, which is provided with a variable valve controller varying a valve profile of an intake valve and/or an exhaust valve in order to improve the power, enhance the fuel economy, and reduce the exhaust emission of the engine. For instance, the variable valve controller changes the control mode of the valves between a low-speed mode and a high-speed mode. In the low-speed mode, a cam which drives the valve is turned into a low-speed cam to reduce the valve lift amount for obtaining a stable combustion. In high-speed mode, the cam is turned into a high-speed cam to increase the valve lift amount for enhancing the power of the engine.
In such a variable valve controller, when the valve mode is not changed properly, the drivability of engine is deteriorated. For instance, when an actual control mode is still in the high-speed mode even though the control mode should be turned into the low-speed mode in a low-load driving condition, the combustion stability is deteriorated to cause the engine stalling. When the actual control mode is still in the low-speed mode even though the control mode should be turned into the high-speed mode in a high-load driving condition, the power of engine is reduced to deteriorate an acceleration thereof.
Under such a situation, diagnosis systems for the variable valve controller have been proposed. Japanese Patent No. 2817055 shows a diagnosis system in which malfunctions of the variable valve controller are detected based on a deviation between an actual valve lift amount sensed by a lift sensor and a reference valve lift amount determined on the basis of the engine speed.
Japanese Patent No. 2571629 shows a diagnosis system in which the valve-close timing is detected by a vibration sensor to be compared with a reference timing, by which malfunctions of the variable valve controller are detected.
JP-4-159426 A shows a diagnosis system in which an intake pipe pressure derived from control conditions of the engine and variable valve controller is compared with an actual intake pipe pressure detected by an intake pipe pressure sensor, by which malfunctions of the variable valve controller are detected.
In the diagnosis system shown in Japanese Patent No. 2817055, it is necessary to newly provide a lift sensor and to keep a mounting space for the lift sensor in a narrow space at vicinity of the valve, which makes the system complicated and increases a cost.
In the diagnosis systems shown in Japanese Patent No. 2571629 and JP-4-159426 A, since the output signals from the pressure sensor and the lift sensor include noises, it is relatively hard to distinguish correct signals from incorrect signals. Therefore, it is difficult to diagnose the variable valve controller correctly.